The present invention relates to a video signal detector for use with a color television receiver wherein an image signal motion between two consecutive frames is detected, and more particularly to a motion detector suitable for detecting a change (motion) of chrominance signal components of an image using a composite color television signal.
In an NTSC system television receiver, the phase of a chrominance sub-carrier is inverted between 1st and 2nd consecutive frames when a video signal of a still image is received. If a composite video signal of a 1st frame delayed by one frame period is added to a composite video signal of a 2nd frame, the chrominance sub-carriers of the 1st and 2nd frames are cancelled out so that a luminance signal can be obtained. If a composite video signal delayed by one frame period is subtracted from a composite video signal of a 2nd frame, a luminance signal is removed and a chrominance signal can be separated. In this manner, in case of a video signal of a still image, cross-components such as cross-color, cross-luminance, i.e., hanging dots are substantially eliminated to enable a high image quality of a television receiver. However, there is no corelation between 1st and 2nd frame video signals of a motion image. Therefore, if such processing of composite video signals between frames is performed when a television receiver receives video signals of a moving image, cross-components contained in the luminance signal or the chrominance signal increase on the contrary. As a result, the quality of an image reproduced on a picture tube deteriorates. In view of this, if there is a motion of an image, the processing of video signals between frames is suspended, and instead in-field processing of video signals is requested, such as processing of video signals between lines. To this end, a detector for detecting a motion of an image is needed. A conventional detector is described in JP-A No. 58-115995.
This detector, however, does not take into consideration a motion detection of video signals whose luminance signal levels do not change between two frames but hues and saturations differ between two frames, namely a detection of video signals having a motion in chrominance. Thus, this detector has a low sensitivity of detecting a motion in chrominance, resulting in large hanging dots over the whole image area having a chrominance motion.